High fracture toughness hydrosilyation cured silicone resin

ABSTRACT

A hydrosilylation reaction curable composition that includes a silsesquioxane polymer, a mixture of silanes or siloxanes as a cross-linking compound and a hydrosilylation reaction catalyst. The curable composition is cured to form a cured resin having high fracture toughness and strength without the loss of elastic modulus.

FIELD OF THE INVENTION

[0001] The invention relates to a cured silsesquioxane resin having highfracture toughness and strength without loss of elastic modulus. Withmore particularity the invention relates to a cured silsequioxane resinthat includes a mixture of silanes or siloxanes as a cross-linkingcompound resulting in an improved fracture toughness.

BACKGROUND OF THE INVENTION

[0002] Silsesquioxane resins have seen increased use in industrialapplications in transportation (automotive, aerospace, naval) and othermanufacturing industries. Silsequioxane resins; exhibit excellent heatand fire resistant properties that are desirable for such applications.These properties make the silsesquioxane resins attractive for use infiber-reinforced composites for electrical laminates, structural use inautomotive components, aircraft and naval vessels. Thus, there exists aneed for rigid silsesquioxane resins having increased flexural strength,flexural strain, fracture toughness, and fracture energy, withoutsignificant loss of modulus or degradation of thermal stability. Inaddition, rigid silsesquioxane resins have low dielectric constants andare useful as interlayer dielectric materials. Rigid silsesquioxaneresins are also useful as abrasion resistant coatings. Theseapplications require that the silsesquioxane resins exhibit highstrength and toughness.

[0003] Conventional thermoset networks of high cross-link density, suchas silsesquioxane resins, typically suffer from the drawback that whenmeasures are taken to improve a mechanical property such as strength,fracture toughness, or modulus, one or more of the other propertiessuffers a detriment.

[0004] Various methods and compositions have been disclosed in the artfor improving the mechanical properties of silicone resins including: 1)modifying the silicone resin with a rubber compound, as disclosed inU.S. Pat. No. 5,747,608 which describes a rubber-modified resin and U.S.Pat. No. 5,830,950 which describes a method of making therubber-modified resin; 2) adding a silicone fluid to a silicone resin asdisclosed in. U.S. Pat. No. 5,034,061 wherein a silicone resin/fluidpolymer is adapted to form a transparent, shatter-resistant coating.

[0005] While the above referenced patents offer improvements in thetoughness of silicone resins, there is an additional need to furtherimprove the toughness of silicone materials for use in high strengthapplications, such as those described above.

[0006] Therefore, it is an object of this invention to provide a processthat may be utilized to prepare a cured silsesquioxane resin having highfracture toughness with minimal loss of modulus.

SUMMARY OF THE INVENTION

[0007] A hydrosilylation reaction curable composition including asilsesquioxane polymer, a mixture of silanes or siloxanes as across-linking compound, and a hydrosilylation reaction catalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] This invention relates to a hydrosilylation reaction curablecomposition that is used to prepare a cured silsesquioxane resin. Thiscurable composition comprises: (A) a silsesquioxane copolymer, (B) amixture of silanes or siloxanes as a cross-linker, (C) a compoundcatalyst, (D) an optional reaction inhibitor and (E) an optionalsolvent.

[0009] Component (A) is a silsesquioxane copolymer comprising units thathave the empirical formula R¹ _(a)R² _(b)R³ _(c)SiO_((4-a-b-c)2),wherein: a is zero or a positive number, b is zero or a positive number,c is zero or a positive number, with the provisos that 0.8 ≦ (a+b+c)<3.0 and component (A) has an average of at least 2 R¹ groups permolecule, and each R¹ is independently selected from monovalenthydrocarbon groups having aliphatic unsaturation, and each R² and eachR³ are independently selected from monovalent hydrocarbon groups andhydrogen. Preferably, R¹is an alkenyl group such as vinyl or allyl.Typically, R² and R³ are nonfunctional groups selected from the groupconsisting of alkyl and aryl groups. Suitable alkyl groups includemethyl, ethyl, isopropyl, n-butyl, and isobutyl groups. Suitable arylgroups include phenyl groups. Suitable silsesquioxane copolymers forcomponent (A) are exemplified by (PhSiO_({fraction (3/2)}))_(.75) (ViMe₂SiO_(½))_(.25), where Ph is a phenyl group, Vi represents a vinyl group,and Me represents a methyl group.

[0010] Component (B) is a mixture of silanes and/or siloxanes thatcontain silicon hydride functionalities that will cross-link with thevinyl group of component (A). The silanes or siloxanes utilized in themixture should have at least two Si-H or silicon hydride functionalitiesand can be represented by the general formula:

[0011] H_(a)R¹ _(b)Si wherein a ≧2 and R¹ is a hydrocarbon for thesilane, and H_(a)R¹ _(b)SicO_((4c-a-b)/2) for the siloxane where a ≧2, b≧4, c ≧2 and R¹ is a hydrocarbon.

[0012] Component B should comprise a mixture of silanes and/or siloxanesthat exhibit a synergistic effect. Such a synergistic effect isexemplified by a cured silsesquioxane resin produced utilizing themixture that has a greater fracture toughness than a cured resinproduced utilizing any of the components of the mixture alone as thecross-linking compound.

[0013] The mixture preferably includes 2 components in which thecomponents range from 20 to 80 molar % of the mixture and even morepreferably from 30 to 70 in molar % of the mixture. An example of apreferred mixture of silanes and siloxanes, is a mixture of diphenylsilane and hexamethyltrisiloxane. Such compounds are commerciallyavailable from Gelast, Inc. of TulIt, Pa. and United ChemicalTechnologies, Inc. of Bristol, Pa.

[0014] Components (A) and (B) are added to the composition in amountssuch that the molar ratio of silicon bonded hydrogen atoms (SiH) tounsaturated groups (C=C) (SiH:C=C) ranges from 1.0:1.0 to 1.5:1.0.Preferably, the ratio is in the range of 1.1:1.0 to 1.5: 1.0. If theratio is less than 1.0: 1.0, the properties of the cured silsesquioxaneresin will be compromised because curing will be incomplete. The amountsof components (A) and (B) in the composition will depend on the numberof C=C and Si--H groups per molecule. However, the amount of component(A) is typically 50 to 80 weight % of the composition, and the amount ofcomponent (B) is typically 2 to 50 weight % of the composition.

[0015] Component (C) is a hydrosilylation reaction catalyst. Typically,component (C) is a platinum catalyst added to the composition in anamount sufficient to provide 1 to 100 ppm of platinum based on theweight of the composition. Component (C) is exemplified by platinumcatalysts such as chloroplatinic acid, alcohol solutions ofchloroplatinic acid, dichlorobis(triphenylphosphine)platinum(II),platinum chloride, platinum oxide, complexes of platinum compounds withunsaturated organic compounds such as olefins, complexes of platinumcompounds with organosiloxanes containing unsaturated hydrocarbongroups, such as Karstedts catalyst (i.e. a complex of chloroplatinicacid with 1,3-divinyl -1,1,3,3-tetramethyldisiloxane) and1,3-diethenyl-1,1,3,3-tetramethyldisiloxane, and complexes of platinumcompounds with organosiloxanes, wherein the complexes are embedded inorganosiloxane resins. A particularly preferred catalyst is a 1%platinum-divinyltetramethyldisiloxane complex commercially availablefrom Chemical Technologies, Inc. of Bristol, Pa.

[0016] Component (D) may include an optional catalyst inhibitor,typically added when a one part composition is prepared. Suitableinhibitors are disclosed in U.S. Pat. No. 3,445,420 to Kookootsedes etal., May 20, 1969, which is hereby incorporated by reference for thepurpose of describing catalyst inhibitors. Component (D) is preferablyan acetylenic alcohol such as methylbutynol or ethynyl cyclohexanol.Component (D) is more preferably ethynyl cyclohexanol. Other examples ofinhibitors include diethyl maleate, diethyl fumamate, bis(2-methoxy-1-methylethyl) maleate, 1-ethynyl-1-cyclohexanol,3,5-dimethyl-1-hexyn-3-ol, 2-phenyl-3-butyn-2-ol, N, N, N′,N′-tetramethylethylenediamine, ethylenediamine, diphenylphosphine,diphenylphosphite, trioctylphosphine, diethylphenylphosphonite, andmethyidiphenylphosphinite.

[0017] Component (D) is present at 0 to 0.05 weight % of thehydrosilylation reaction curable composition. Component (D) typicallyrepresents 0.0001 to 0.05 weight % of the curable composition. Component(D) preferably represents 0.0005 to 0.01 weight percent of the totalamount of the curable composition. Component (D) more preferablyrepresents 0.001 to 0.004 weight percent of the total amount of thecurable composition.

[0018] Components (A), (B), (C) and (D) comprise 10 to 99.9 weight % ofthe composition. The composition may further comprise one or moreoptional components such as reaction inhibitors, processing additives orother components known in the art.

[0019] The hydrosilylation reaction curable composition comprisingcomponents (A), (B), and (C), and any optional components can bedissolved in component (E), an optional solvent. Typically, the amountof solvent is 0 to 90 weight %, preferably 0 to 50 weight % of thecurable composition. The solvent can be an alcohol such as methyl,ethyl, isopropyl, and t-butyl alcohol; a ketone such as acetone,methylethyl ketone, and methyl isobutyl ketone; an aromatic hydrocarbonsuch as benzene, toluene, and xylene; an aliphatic hydrocarbon such asheptane, hexane, and octane; a glycol ether such as propylene glycolmethyl ether, dipropylene glycol, methyl ether, propylene glycol n-butylether, propylene glycol n-propyl ether, and ethylene glycol n-butylether; a halogenated hydrocarbon such as dichloromethane,1,1,1-trichloroethane and methylene chloride; chloroform; dimethylsulfoxide; dimethyl formamide; acetonitrile and tetrahydrofuran. Apreferred solvent is toluene.

[0020] There is also disclosed a process for preparing a hydrosilyationreaction curable composition comprising the steps of:

[0021] a) providing a silsesquioxane polymer;

[0022] b) providing a mixture of silanes or siloxanes as a cross-linkingcompound;

[0023] c) mixing the components of a), and b) to form a curablecomposition;

[0024] d) adding a hydrosilylation reaction catalyst to the curablecomposition of step c)

[0025] e) adding an optional reaction inhibitor to the catalyst of stepd) before or after mixing the reaction catalyst with the curablecomposition;

[0026] f) curing the curable composition of step e) to form a curedresin having high fracture toughness.

[0027] The silsesquioxane polymer, as described previously, is firstmixed with the cross-linking compound, as disclosed above. After thecomponents above are mixed, the hydrosilylation catalyst is mixed intothe composition and the mixture is poured into a mold. The mixing of thecurable composition of the present invention may also include the stepof degassing the composition before curing. Degassing is typicallycarried out by subjecting the composition to a mild vacuum.

[0028] The mold is then subjected to the following curing steps: 1)curing the curable composition in the mold at a temperature of 85° C.for 24 hours, 2) curing the curable composition in the mold at atemperature of 150° C. for 24 hours, 3) curing the curable compositionin the mold at a temperature of 200° C. for 24 hours.

[0029] It should be realized that the silicone resins mixed with anySi-H functional cross-linkers can be used as a continuous phase forfiber reinforced composites. Such fiber reinforcements can include anycommon reinforcement fibers such as quartz, glass graphite, etc.

EXAMPLES

[0030] The following examples are intended to illustrate the inventionto those skilled in the art and should not be interpreted as limitingthe scope of the invention as set forth in the appended claims.

[0031] Reference Example 1

[0032] Three Point Flexural Testing

[0033] The three point bending test was performed on an Instron 4500 perASTM standard D 790-961. The cured resin specimens prepared in theExamples described below were cut into 5.08 cm X 1.27 cm specimens usinga band saw. The specimens were then machined to a thickness of 0.25 cm.The specimens were tested using a cross head speed of 1 mm/min and asupport span of 3.81 cm.

[0034] During testing, force-displacement curves were recorded. Thetoughness of the cured resin was obtained as the area under thestress-strain curves. The flexural strength was calculated using thepeak force as:

S=3PL/2bd ²

[0035] where Ε is the stress in the outer surface at the mid span, P themaximum load, L the support span, and b and d are the width andthickness of the beam. The maximum strain was calculated, using themaximum displacement, as:

Ε=6Dd/L ²

[0036] where Ε is the strain at break and D is the maximum displacement.The slope of the steepest initial straight-line portion of theload-displacement curve was taken as the Young's modulus.

[0037] Reference Example 2

[0038] Fracture Toughness Testing

[0039] The plane strain fracture toughness, K_(lc), was obtained perASTM D 5045-96, and the critical strain energy release rate, G_(lc), wascalculated from K_(lc)based on Linear Elastic Fracture Mechanics (LEFM)assumptions. 5.08 cm X 0.95 cm samples were cut using a band saw and anotch was cut at the center of the specimen. A natural crack extendingfrom the root of the notch to about half of the width was produced bygently tapping a sharp razor blade into the notch. Samples wereconditioned at 73° C. for at least twenty-four hours before testing toallow full relaxation of deformation. The displacement rate of the testwas 10 mm/minute with a support span of 3.81 cm.

[0040]K _(lc)=(P/(BW ^(½)))f(x)

[0041] where P is the highest load and:

f(x)=6x ^(½) (1.99−x(1−x)(2.15−3.93x+2.7x²))/((1+2x)(1−x)^({fraction (3/2)})

[0042] where x is the pre-crack to specimen width ratio, a/W. After thetest the pre-crack length was measured. Only those specimens with avalue between 0.45 to 0.55 were considered valid. The variation of xacross the thickness should be less than 10%. The validity of the testwas further ensured by comparing the sample dimensions with theestimated plastic zone size enlarged by approximately 50:

B,a,(W−a)<2.5(K _(lc)/Υ_(γ))²

[0043] where yy is the yield stress of the sample.

[0044] G_(lc) was calculated by:

G _(lc) =K ² _(lc) (1−v ²)/E

[0045] where upsilon, the Poisson's ratio of the resin, was neglected tosimplify the experiment. For a glassy polymer with a Poisson's ratio of0.3, G_(lc) was exaggerated by about 9%. However, the relative rankingof G_(lc) values would not be obscured since the change of the square ofthe Poisson's ratio is usually small from one resin to another ofsimilar stiffness.

[0046] Reference Example 3

[0047] Dynamic Mechanical Analysis

[0048] Dynamic mechanical analysis was carried out on a Seiko DynamicMechanical Rheology Station DMS 200. A specimen 20 mm long, 4 mm wideand 1 mm thick was mounted in two grips 14 mm apart. The specimen wasthen subjected to a sinusoidal tensile displacement at a frequency of 1Hertz. The tension was measured and the storage and loss moduli and theloss factor were calculated. The tests were performed at temperaturesranging from −150 to 350° C. All tests were performed in a nitrogenenvironment with a gas flow rate of 200 ml/min.

EXAMPLE 1

[0049] The silsesquioxane resin(PhSiO_({fraction (3/2)}))_(0.75)(ViMe₂SiO_(½))_(0.25) was utilized asthe base resin for forming a cured resin.

[0050] A mixture of two silicon hydride (hexamethyltrisiloxane anddiphenylsilane) containing cross-linking agents was utilized in varyingmolar ratios as detailed in Table 1. Table 1 indicates the amount ofeach component added in grams, and also indicates the molar % of thetotal mixture of the cross-linking agents utilized. D indicatesdiphenylsilane and P indicates hexamethyltrisioxane in Table 1.

[0051] The silsesquioxane resin was a 75% solution in toluene;therefore, to reduce the amount of toluene, the resin was heated in avacuum oven under 50 mmHg at 50-60° C. for 30 minutes. Approximately40-50% of the toluene was removed as a result of the process.

[0052] The mixture of cross-linking agents was first added to theprocessed silsesquioxane resin, and then a hydrosilyation reactioncatalyst of a 2.09 wt% platinum complex of vinyl terminatedpolydimethylsiloxane was added.

[0053] The mixture was stirred vigorously for 5 to 10 minutes and thentransferred to a mold. The mold was degassed in a vacuum oven at roomtemperature under 50 mmHg for 10 minutes to remove trapped air from themixing step.

[0054] The mold was then moved to an air circulating oven and subjectedto the wing curing sequence: 85° C. for 24 hours, 150° C. for 24 hours,2000 for 24 hours. After the final step, the casting was removed fromthe oven for testing. The results of the tests for each sample aredisplayed in table 2. TABLE 1 Recipes for Various Cross-linked MolarRatios Component D/P molar Catalyst SAMPLE A(g) D(g) P(g) ratio (ppm) 160 14.33 0 100/0   1 2 60 10.03 3.8 70/30 30 3 60 7.17 6.33 50/50 30 460 4.3 8.86 30/70 30 5 60 0 12.65  0/100 30

[0055] TABLE 2 Young's Flexural Flexural K_(IC) ^(1/2) SAMPLE Modulus(KSi) Strength (KSi) Strain (Mpam) G_(IC)(J/M²) 1  76.2 (3.1)  2.70(0.21) 10.27 (1.20)  0.58 (0.05) 582.25 (17.1)  2 137.78 (5.22) 4.23(0.14) 8.63 (0.78)  .72 (0.05)  87.68 (10.55) 3 167.54 (5.41) 5.06(0.21) 7.31 (0.37) 0.49 (0.04) 210.23 (34.34) 4  201.2 (6.4)  6.00(0.29) 5.94 (1.16) 0.47 (0.03) 155.02 (11.43) 5   204 (7.37) 6.89 (0.29)7.35 (0.96) 0.35 (0.02) 550.72 (69.73)

[0056] As can be seen from the above results, a molar ratio of 70/30 ofD to P exhibits acture toughness value K_(lc) of 0.72 MPam^(½) which ishigher than either of D or P e, with values of K_(lc) of 0.35 and 0.58,respectively.

[0057] While a preferred embodiment is disclosed, a worker in this artwould understand that various modifications would come within the scopeof the invention. Thus, the following claims should be studied todetermine the true scope and content of this invention.

What is claimed is:
 1. A hydrosilylation reaction curable compositioncomprising: a) a silsesquioxane polymer b) a mixture of silane and/orsiloxane cross-linking compounds; and c) a hydrosilylation reactioncatalyst; the cured composition displaying a fracture toughness that isgreater than the fracture toughness of a cured composition formedutilizing any of the individual cross-linking compounds of the mixturealone.
 2. The hydrosilylation reaction curable composition of claim 1wherein the silsesquioxane resin comprises a copolymer resin.
 3. Thehydrosilylation reaction curable composition of claim 2 wherein thecopolymer resin comprises, a copolymer resin having the empiricalformula R¹aR²bR³cSiO_((4-a-b-c))/2, wherein: a is zero or a positivenumber, b is zero or a positive number, c is zero or a positive number,with the provisos that 0.8 ≦(a+b+c) ≦3.0 and component (A) has anaverage of at least 2 R¹ groups per molecule, and each R¹ isindependently selected from monovalent hydrocarbon groups havingaliphatic unsaturation, and each R² and each R³ are independentlyselected from monovalent hydrocarbon groups and hydrogen.
 4. Thehydrosilylation reaction curable composition of claim 3 wherein thesilsesquioxane resin comprises (PhSiO_({fraction (3/2)}))_(.75) (ViMe₂SiO_(½))_(.25), where Ph is a phenyl group, Vi represents a vinyl group,and Me represents a methyl group.
 5. The hydrosilylation reactioncurable composition of claim 1 wherein the mixture of cross-linkingcompounds is formed of silanes or siloxanes having at least two siliconhydride functionalities.
 6. The hydrosilylation reaction curablecomposition of claim 5 wherein the mixture of cross-linking compoundscomprises diphenylsilane and hexamethyltrisiloxane.
 7. Thehydrosilylation reaction curable composition of claim 6 wherein themixture comprises from 20 to 80 mole % diphenylsilane based on the totalmoles of crosslinkers with the remainder being hexamethyltrisiloxane. 8.The hydrosilylation reaction curable composition of claim 7 wherein themixture comprises 70 mole % diphenylsilane and 30 mole %hexamethyltrisiloxane.
 9. The hydrosilylation reaction curablecomposition of claim 1 further including a reaction inhibitor.
 10. Aprocess for preparing a hydrosilyation reaction curable compositioncomprising the steps of: a) providing a silsesquioxane polymer; b)providing a mixture of silanes or siloxanes as a cross-linking compound;c) mixing the components of a), b), to form a curable composition; d)adding a hydrosilylation reaction catalyst to the curable composition ofstep c) e) curing the curable composition of step d) to form a curedresin having a fracture toughness that is greater than the fracturetoughness of a cured composition utilizing any of the individualcross-linking compounds of the mixture alone.
 11. The process of claim10 wherein the curing step includes the steps of: 1) curing the curablecomposition in the mold at a temperature of 85° C. for 24 hours, 2)curing the the curable composition in the mold at a temperature of 150°C. for 24 hours, 3) curing the curable composition in the mold at atemperature of 200° C. for 24 hours.
 12. The process of claim 10 whereinthe silsesquioxane resin comprises a copolymer resin.
 13. The process ofclaim 12 wherein the copolymer resin comprises, a copolymer resin havingthe empirical formula R¹ _(a)R² _(b)R³ _(c)SiO_((4-a-b-c)/2), wherein: ais zero or a positive number, b is zero or a positive number, c is zeroor a positive number, with the provisos that 0.8 ≦(a+b+c) ≦3.0 andcomponent (A) has an average of at least 2 R¹ groups per molecule, andeach R¹ is independently selected from monovalent hydrocarbon groupshaving aliphatic unsaturation, and each R² and each R³ are independentlyselected from monovalent hydrocarbon groups and hydrogen.
 14. Theprocess of claim 13 wherein the silsesquioxane resin comprises(PhSiO_({fraction (3/2)}))_(.75) (ViMe₂ SiO_(½))_(.25), where Ph is aphenyl group, Vi represents a vinyl group, and Me represents a methylgroup.
 15. The process of claim 10 wherein the mixture of cross-linkingcompounds comprises: a silane having at least two silicon hydridefunctionalities or a siloxane having at least two silicon hydridefunctionalites.
 16. The process of claim 15 wherein the cross-linkingmixture of compounds comprises diphenylsilane and hexamethyltrisiloxane.17. The process of claim 10 further including the step of adding areaction inhibitor.